Power supply switching circuits for providing power to electronic circuits or load devices are well known in the art. The switching circuit is coupled to primary and secondary power supply sources. The circuit compares the voltage of the primary power supply against a reference voltage, and switches the load to the secondary power supply when the primary supply voltage falls below the reference voltage.
A typical application of the power supply switching circuit is in the area of portable electronic devices such as notebook computers. The computers generally have line voltage or a rechargeable battery as the primary power supply source. The secondary, power supply source is a back-up battery, such as a lithium battery, for providing power to the computer when the rechargeable battery voltage falls below an operation voltage level. The power supply switching circuit monitors the voltage level of the rechargeable battery and switches to the back-up battery widen the rechargeable battery sufficiently drains to fall below the reference voltage.
Typically, integrated circuit (IC) chips in a computer have a normal operating voltage of 5 volts or 3.2; volts. Hence the rechargeable battery, when fully charged, generates 5 volts, 3.3 volts or the like. The backup battery such as the lithium battery generates a voltage level in the range of 3-3.5 volts. For the 5 volt IC chips, the voltage level of the back-up battery is not ideal but is sufficiently high to operate the chips for a short period of time. IC chips that require 5 volts are not ideally suited for portable applications because they draw relatively high current and run relatively hot. For these reasons, the semiconductor industry have started to produce IC chips that have a normal operating voltage of 3 volts or less. Because the 3 volt operating voltage of those IC chips is below the back-up battery voltage, it has caused a problem in the operation of the prior art switching circuit. When the switching circuit is providing power from the rechargeable battery, a parasitic diode in the switching circuit may become forward biased and drain power from the back-up battery. The problem may cause shorter life for the back-up battery, and may even lead to a system failure of the load device being driven. This occurs when the switching circuit switches to the back-up battery when it has discharged below the minimum operating voltage of the load device.
Therefore, it is desirable to provide a power supply switching circuit that prevents unnecessary current drain from the back-up battery when the normal operational voltage of the circuit is below the back-up battery voltage.